Depletion of fossil fuel resources such as petroleum or coal as industrial raw materials has been expected, and thus much attention has been focused on biomass permanently used. Of the biomass, lignocellulosic biomass is highly useful because it does not compete with other food resources.
Lignocellulosic biomass is composed of hydrophilic carbohydrate such as cellulose and hydrophobic lignin. The hydrophilic carbohydrate such as cellulose is used as a raw material for paper, but it can be used as a sugar supply source through a hydrolytic process. The hydrophobic lignin is a natural phenolic polymer which accounts for 15˜20% of lignocellulosic biomass, and its quantitative, qualitative molecular control is difficult due to a complicated structure. Further, the hydrophobic lignin forms a complicated complex with the hydrophilic carbohydrate such as cellulose, and therefore, there is a restriction in use.
To increase usability of the hydrophobic lignin, the hydrophobic lignin can be converted into an organic solvent-soluble form by avoiding lignin recondensation and increasing the content of a phenolic —OH group by treatment of lignocellulosic biomass with a phenol-based compound and a strong acid, but a molecular weight of the resulting lignin is not as low as those of petrochemical raw materials.
With regard to use of the degraded lignin derivative as a starting material of other chemical reactions such as preparation of commercial polymer resins, the lignin derivative becomes more useful as it has a low molecular weight.
Accordingly, there is a need for a method capable of easily degrading lignin into a low-molecular weight lignin with a high yield and a conversion method of maintaining a low equivalent weight of phenolic OH of the derived lignin (an equivalent weight of phenolic OH=a molecular weight of lignin/the number of phenolic OH).